(Not Applicable)
1. Field of the Invention
The invention relates generally to epitaxial metal oxide buffer layers on metal substrates and articles made therefrom. More specifically, the invention relates to a vacuum process for depositing epitaxial layers of oxides on biaxially-textured metal substrates and superconducting layers on the epitaxial layers, and articles made therefrom.
2. Description of Related Art
Epitaxial metal oxide buffer layers on substrates with crystalline, polycrystalline, or biaxially-textured metal surfaces are potentially useful where an electronically active layer is deposited on the buffer layer. The electronically active layer may be a superconductor, a semiconductor, or a ferroelectric material. For example, a biaxially-textured superconductor article to be used for power transmission lines has a multi-layer composition 10, as in FIG. 1.
Such deposited superconductor articles most commonly consist of a biaxially-textured metal surface 12, a plurality of buffer layers 14, 16, and a superconducting layer 18. The biaxially-textured metal surface 12, most commonly formed from Cu, Ag, Ni, or Ni alloys, provides support for the superconductor article, and can be fabricated over long lengths and large areas. Epitaxial metal oxide buffer layers 14, 16 comprise the next layers in the superconductor article. The buffer layers 14, 16 are commonly formed from Y2O3 or CeO2, and serve as chemical barriers between the metal surface 12 and the last layer, which is the superconducting layer 18.
Current materials research aimed at fabricating improved high-temperature superconductor articles is largely focused on epitaxial growth of high-temperature superconducting layers on biaxially-textured metal surfaces. Superconducting articles with current densities (Jc) in excess of 0.1 MA/cm2 at 77 K have been achieved for epitaxial YBa2Cu3O7 films on biaxially-textured Ni or Ni-based alloy surfaces with the use of certain epitaxial buffer layer constructs between the metal surface and the superconducting layer. In previous work, the synthesis of high-temperature superconductor layers capable of carrying a high (at least 0.1 MA/cm2 at 77K) Jc has required the use of complex, multilayered buffer architectures.
A biaxially-textured article can be defined as a polycrystalline material in which the crystallographic in-plane and out-of-plane grain-to-grain misorientations are small (typically less than 40 degrees) but finite (typically greater than 2 degrees). In order to realize a high-temperature superconducting layer, such as YBa2Cu3O7, possessing a Jc greater than approximately 0.1 MA/cm2 at 77K on a biaxially-textured metal substrate, the buffer layer architecture should be epitaxial relative to the metal substrate and crack-free. Most preferably, the grains of the buffer layer should be crystallographically aligned perpendicular to the plane of the metal substrate (c-axis oriented) and parallel to the plane of the metal substrate (a-b alignment).
Formation of superconductor articles with this orientation begins with the selection of the metal surface 12. The crystallographic orientation of the metal surface 12 is preferably maintained in the buffer layers 14, 16 and the superconducting layer 18, to the maximum extent possible. Numerous conventional processes are currently being used to grow buffer layers 14, 16 on a metal substrate 12. These processes include vacuum methods, such as pulsed laser deposition, vapor deposition, and sputtering.
In addition to being epitaxial relative to the biaxially-textured metal surface, layers are preferably chemically compatible with both the metal surface superconductor, and mechanically robust so as to prevent microscopic crack formation in the high-temperature superconducting layer and the buffer layers. Prior to the present invention, buffer layers that met these objectives required multilayer combinations of various oxides. For example, CeO2 has been used to nucleate an epitaxial (001) oriented oxide layer on a biaxially textured (100) Ni surface. A tendency for the CeO2 layer to crack due to differences in the thermal expansion coefficients of the oxide film and the superconductor layer requires an additional epitaxial yttria-stabilized zirconia (YSZ) buffer layer on the CeO2 in order to achieve crack-free superconductor articles. In this arrangement, the superior mechanical properties of the YSZ layer circumvent the microcracking problem, and enable the formation of superconducting layers with a high Jc. The CeO2 layer serves primarily to nucleate a (001) oriented epitaxial oxide on the metal surface.
Though effective in forming a high Jc superconductor article, the use of a multilayer buffer architecture, as opposed to a single layer buffer architecture, increases the complexity of the superconductor article fabrication process. Using multiple buffer layers typically requires the use of additional raw materials, as compared to a single buffer layer architecture. In addition, having CeO2 as the nucleating layer tends to permit the formation of microscopic cracks that can limit the maximum Jc of the superconductor article.
Epitaxial (001) ZrO2, HfO2, or related compounds having Ca or a rare earth element grown directly on a biaxially-textured (001) metal surface, such as a Ni or Ni-based alloy substrate, has been an attractive candidate for an improved single layer buffer architecture, as these materials are mechanically-robust oxides. Unfortunately, efforts to grow these epitaxial layers with a (001) orientation directly on such biaxially-textured (001) metal substrates have been unsuccessful. Specifically, such efforts have resulted in an undesirable mixture of (100) and (111) orientations.
Epitaxial ZrO2, HfO2, or related oxides on crystalline or polycrystalline metal surfaces have potential application in fields other than superconductors. Epitaxial ZrO2 or HfO2 on crystalline metal surfaces may prove useful where thin epitaxial layers are needed in electronic applications. Furthermore, epitaxial oxide layers on polycrystalline metal surfaces have potential use in tribological or fuel cell applications where the properties of the metal/oxide interface largely determine material performance. For epitaxy on randomly-oriented polycrystalline metal surfaces, the epitaxial relationship involves a grain-by-grain registry of film and substrate crystallographic orientations.
For further information, refer to the following publications:
1. D. P. Norton, A. Goyal, J. D. Budai, D. K. Christen, D. M. Kroeger, E. D. Specht, Q. He, B. Saffian, M. Paranthaman, C. E. Klabunde, D. F. Lee, B. C. Sales, and F. A. List, xe2x80x9cEpitaxial YBa2Cu3O7 on Biaxially Textured Nickel (001): An Approach to Superconducting Tapes with High Critical Current Density,xe2x80x9d Science 274, 755 (1996).
2. M. Paranthaman, A. Goyal, F. A. List, E. D. Specht, D. F. Lee, P. M. Martin, Q. He, D. K. Christen, D. P. Norton, J. D. Budai, and D. M. Kroeger, xe2x80x9cGrowth of Biaxially Textured Buffer Layers on Rolled-Ni Substrates by Electron Beam Evaporation,xe2x80x9d Physica C 275, 266 (1997).
The invention relates to an article with an improved buffer layer architecture. An epitaxial article, according to the invention, comprises a substrate having a metal surface, and a single epitaxial layer on the surface of the substrate. The single epitaxial layer comprises at least one of the group consisting of ZrO2, HfO2, and compounds having at least one of Ca and a rare earth element stabilizing cubic phases of ZrO2 and/or HfO2. The article can also include a superconducting layer deposited on the single epitaxial layer.
A method for preparing an epitaxial article, according to the invention, comprises the steps of providing a substrate with a metal surface, depositing a single epitaxial layer comprising at least one material selected from the group consisting of ZrO2, HfO2, and compounds having at least one of Ca and a rare earth element stabilizing cubic phases of at least one of ZrO2 and HfO2, wherein the epitaxial layer depositing step occurs in a vacuum with a background pressure of no more than 1xc3x9710xe2x88x925 Torr. The method can further comprise the step of depositing a superconducting layer on the single epitaxial layer.
An epitaxial article, according to the invention, comprises a substrate having a metal surface, and an epitaxial buffer layer on the surface of the substrate. The epitaxial buffer layer comprises at least one material selected from the group consisting of ZrO2, HfO2, and compounds having at least one of Ca and a rare earth element stabilizing cubic phases of at least one of ZrO2 and HfO2. The epitaxial article can also include an epitaxial capping layer on the epitaxial buffer layer, where the epitaxial capping layer is of a different composition than the epitaxial buffer layer, and a superconducting layer deposited on the epitaxial capping layer.
According to the invention, a method for preparing an epitaxial article comprises the steps of providing a substrate with a metal surface, and depositing an epitaxial buffer layer comprising at least one material selected from the group consisting of ZrO2, HfO2, and compounds having at least one of Ca on a rare element stabilizing cubic phases or ZrOor HfO2, wherein the epitaxial buffer layer depositing step occurs in a vacuum with a background pressure of no more than 1xc3x9710xe2x88x925 Torr. The method can also include the steps of depositing an epitaxial capping layer on the epitaxial buffer layer, where the epitaxial capping layer is of a different composition than the epitaxial buffer layer, and depositing a superconducting layer on the epitaxial buffer layer.
The metal surface of the substrate can be crystalline or biaxially-textured. If the metal surface is biaxially-textured, it can be a rolled and annealed biaxially-textured metal surface. The metal surface can comprise at least one metal selected from the group consisting of Cu, Cu-based alloy, Ag, Co, Mo, Cd, Pt, Pd, Ni, and Ni-based alloy. Where the metal surface comprises at least one metal selected from the group consisting of Ni and Ni-based alloys, the metal surface can be alloyed with at least one alloying agent selected from the group consisting of Co, Cr, V, Mo, W, and rare earth elements.
The epitaxial buffer layer can comprise at least one material selected from the group consisting of ZrO2 and HfO2 sub-units in the crystalline lattice of the epitaxial buffer layer. Alternatively, the epitaxial buffer layer can comprise at least one material having the structure ABO3, where A is selected from the group consisting of Ba, Ca, and Sr, and B is selected form the group consisting of Zr and Hf. Preferably, the epitaxial buffer layer is composed of YSZ.
The epitaxial capping layer can comprise at least one material which is a rare earth oxide, such as CeO2 and Y2O3. The epitaxial capping layer can reduce the lattice mismatch between the superconducting layer and the epitaxial buffer layer. The superconducting layer can comprise REBaCu3O7, where RE is a rare earth element. Preferably, the superconducting layer comprises YBaCu3O7.